The present disclosure relates to an image forming apparatus for transferring a toner image to a sheet and particularly to an image forming apparatus capable of transferring a toner image to a long sheet larger than A3 size.
An image forming apparatus such as a copier, a printer or a facsimile machine utilizing an electrophotographic method forms a toner image on an image bearing member (e.g. photoconductive drum or transfer belt) by supplying a developer to an electrostatic latent image formed on the image bearing member and developing the electrostatic latent image. A touch-down developing method using a two-component developer containing nonmagnetic toner particles and magnetic carrier particles is known as one of developing methods. In this method, a two-component developer layer (so-called magnetic brush layer) is carried on a magnetic roller, the toner particles are received from the magnetic brush layer and a toner layer is carried on a developing roller, and the toner particles are supplied from the toner layer to the image bearing member, thereby visualizing the electrostatic latent image.
In a developing device adopting the touch-down development method, it is known to perform a stripping operation of forcibly collecting toner particles once carried on the developing roller by the magnetic brush layer on the magnetic roller by changing a bias applied to the developing roller every time one sheet is printed. By performing this stripping operation, it is possible to prevent the deterioration of the toner particles associated with the stay of the toner particles on the developing roller for a long time.
Some of image forming apparatuses can print not only standard sized sheets such as A4 and A3 sheets, but also long sheets, the size of which in a sub scanning direction is 1000 mm or longer. Since a developing time per sheet becomes longer in printing such long sheets, a toner layer is carried on a developing roller for a longer time. Thus, even if the stripping operation is performed between sheets, the toner particles on the developing roller may be excessively charged during a transfer process for one long sheet and a transfer failure (image defect) such as a solid image blank area may occur.
An object of the present disclosure is to prevent the occurrence of an image defect associated with the deterioration of toner particles in an image forming apparatus capable of transferring a toner image to a long sheet.